Simple, solitary, unicameral bone cysts (UBCs) are common lesions of unknown cause that occur in the upper humerus or femur. Theories of etiology include a defect in enchondral bone formation or altered hemodynamics with venous obstruction, causing increased interosseous pressure and cyst formation. The cysts are filled with yellow fluid and lined with a fibrous capsule.
UBCs are most often first diagnosed when complicated by pain or a pathologic fracture. Their radiographic appearance is usually characteristic. The lesions are usually metaphyseal, expand the bone, have well-defined margins, evoke little reaction, and appear cystic with irregular septa.
Active cysts abut the growth plate and occur in children less than 10 to 12 years of age. They are more likely to recur after treatment and are associated with growth arrest that may follow a fracture.
Inactive cysts are separated from the plate by normal bone and usually occur in adolescents over 12 years of age.
Fractures are usually the presenting complaint. Sometimes the fracture line is difficult to separate.
Management is complicated by recurrence. The usual natural history of these cysts is to become asymptomatic following skeletal maturation. The objective of treatment is to minimize the disability when cysts are likely to fracture. These lesions are not precancerous.
Humeral cysts Place the child in a sling to allow the fracture to heal and to reestablish stability. Seldom does the effect of the trauma result in permanent healing of the cyst. Plan to manage the cyst by a series of injections with steroid, bone marrow, or bone matrix. Some doctors recommend breaking up the adhesion by forceful injections or perforating the septa with a trochar. Recurrence can be managed by repeated injections or curettage and grafting with autogenous or bank bone. Opinions differ regarding how aggressively recurrence is managed.
Femoral cysts are much more difficult to manage because of the load carried by the femur. Plan to curette and graft the cyst and stabilize the fracture with flexible intramedullary fixation. Complications include malunion with coxa vara and avascular necrosis with displaced neck fractures. This fixation is permanent and may prevent additional fractures even if some recurrent cyst formation occurs. An alternative approach is injection followed by spica cast protection for 6 weeks.
Calcaneal cysts are best managed by curettage. Small lesions may be treated by injection.
Operative Management of Bone Cysts
Select management based on the child’s age; location, size, and position of the cyst in the bone; and previous treatment.
Selection of treatment method
Generally, injection treatment is appropriate for cysts of nonweight-bearing bones. Cysts of the proximal femur and large cysts in other sites of the lower limb may be managed by curettage and grafting.
Confirm the diagnosis by inserting a #18 spinal needle into the cyst through the thinnest wall. Aspirate the cyst. Simple bone cysts are filled with clear yellow fluid. It may be slightly blood-tinged due to procedure. If no fluid is obtained, the lesion may be a solid tumor. Consider biopsy.
Injection treatment for unicameral cyst
If the cyst contains yellow fluid, perform a cystogram. Inject diluted contrast solution such as renografin to determine whether the cyst has single or multiple chambers. The options for injection treatment include:
Steroid Inject 50–100 mg of methyl methylprednisolone based on the cyst size.
Autogenous bone marrow Aspirate about 50 cc from the posterior iliac crest. Mix with bone collagen slurry and inject the combination into the cyst.
Treatment of multilocular cysts
If only part of the cyst fills during the arthrogram, modify the plan.
Injection Inject each cavity with steroids or marrow or break up the cyst.
Mechanical breakup of septa Percutaneously introduce a trochar into the cyst and break up the septa. Inject steroid or marrow as described above.
The arthrogram often shows a venogram. The cyst may fill completely, or incompletely.
Manage cysts in weight-bearing bones, especially the upper femur, by curettage and grafting. Following thorough curettage, fill cyst with autogenous or bank bone. Some recommend supplementing the curettage by freezing with liquid nitrogen, phenolization, or burning with an argon laser. The value of these supplemental measures has not been confirmed. Stabilize proximal femoral lesions with IM rods or by nail-plate fixation.
Recurrence and growth arrest of the upper humeral physis are most common. Growth arrest is usually due to the lesion, and usually following a fracture, not the surgery. Recurrence is common and requires long-term thoughtful management.
Aneurysmal Bone Cysts
An aneurysmal bone cyst is considered to be a pseudotumor possibly secondary to subperiosteal or interosseous hemorrhage or a transitional lesion secondary to some primary bone tumor.
The diagnosis can usually be established by a combination of the location of the lesion, the age of the patient, and the appearance on conventional radiographs. ABCs are eccentric, expansile, cystic lesions with a high recurrence rate. Lesions present in a variety of patterns and are sometimes difficult to differentiate from simple bone cysts.
Activity of the lesion The activity level can also be assessed by the appearance of the lesion’s margins.
Inactive cysts have intact, well-defined margins.
Active cysts have incomplete margins but the lesion is well defined.
Aggressive cysts show little reactive bone formation and poorly defined margins.
Other imaging is often necessary, especially in aggressive cysts. Fluid levels are common and can be seen on CT scans and MRI studies.
Spine About 10–30% of ABC lesions are in the spine. They most commonly occur in the cervical and thoracic levels. Lesions arise in the posterior elements but may extend to involve the body. Study posterior elements with CT and MRI preoperatively. The possible need for a combined approach, complete excision, and stabilization, as well as the risk of recurrence, complicates management.
Long bones Options include complete excision or saucerization, leaving a cortical segment intact, or curettage with cryotherapy or with a mechanical burr.
Pelvis Manage most lesions by curettage and bone grafting. Some recommend selective embolization. Be prepared for extensive blood loss.
Complications Bleeding can pose a significant problem.
Recurrence may require more aggressive management that might include more extensive excision. Expect a recurrence rate of 20–30% following curettage.